The aim of the proposed research is to study by indirect calorimetry the adjustments in energy expenditure which serve to stabilize body weight at a particular level. It is our hypothesis that these metabolic adjustments are important not only to the maintenance of normal body weights but that they serve a similar role in the maintenance of abnormally high (or low) body weights. We propose to examine these metabolic adaptations in normal weight, obese (VMH-lesioned, Zucker obese, and dietary obese), and lean(LH-lesioned) rats with the object of assessing in each preparation the role of energy expenditure in maintaining their body weight at these different levels. A further concern will be to determine which categories of energy expenditure (e.g., the energy costs for basal metabolism, for processing ingested food, or for spontaneous motor activity) are most involved in these regulatory adjustments.